Peelbone,
Before you tackle the crossover between the subs and mains, I'd suggest you first time alignment everything because how a sound source drives a room is dependent on its location. Find and settle on your mains locations first. Then focus on the subs. Because you are using a 2 channel DAC you are limiting yourself in ways that a multi-channel DAC would not. With a 2 channel DAC here are some strategies that I would consider:
(A) Use the subs own delay dial to time align the left sub with the left main. Due to the subs inherent group delay you will need to place the sub closer to your listening position than the mains are and then experiment with the delay settings on the sub. Instead of just relying on your ears for time alignment, you might want to use Audiolense (AL) - focusing on the left channel for example, unplug the right main and right sub from your preamp so that they are off, then plug your left sub into the right preamp output. Run a 2 channel sweep and note the left and right channel delay values. If the subs delay value is greater than the mains then you have to reduce the subs delay amount and if you get to 0 delay on the sub setting and its AL delay value is still greater than the main, then you will have to move the sub closer to the mic. When subs AL delay equals the left mains then they are time aligned. Repeat process for right channel.
(B) Do not use the subs delay but instead use physical location to time align the sub with the main. If after running a AL sweep the subs AL delay is greater than its closest main speaker (using the same trick of unplugging the right channel main+sub and plugging the left sub into the right preamp input) then keep moving the sub closer to the mic until the AL delay values for main and sub are the same or as close as you can get them for time alignment. With a multi-channel DAC you can stick the subs pretty much anywhere and AL will automatically time align them. This has the advantage of placing the subs where they drive the room the best which may be behind the mains so AL would delay the mains so the subs can catch up to them in this scenario.
Now with each sub time aligned I would then focus on the subs low pass crossover on its plate amp; do not touch the sub delay knob as you will undue all your hard work to this point. Just stick to the subs volume and crossover frequency and polarity switch if it has one. If you try to use AL to high-pass the mains then your Y-adapter is passing along the high-passed signal to the subs and they will not be playing the right frequencies they were meant to play. So run your mains without a high pass crossover or set the high pass setting really low like 5Hz so that the subs still receive their correct frequencies. Then experiment with the subs crossover settings and slopes if your sub provides different slope values. Rerun an AL sweep each time you change the subs crossover frequency (or slope) and filter the results in AL so you can see the left main and left sub together (the right channel will show the left sub) and click the button to view both channels together so you can see their overlap. Once you are happy repeat the process for the right channel.
Then connect the left and right channels to your preamp as normal with Y-adapters feeding your subs I presume. Now run AL sweep for the combined left main+left sub and right main+right sub and pick your target curve and see what is generated as a predicted response curve. Examine the Corrections curves to make sure that there is not any one peak much taller than any others.
Hope this helps . . .
